The position and orientation errors of a manipulator are primarily caused by deviations of geometric and non-geometric parameters from their nominal dimensions. Geometric errors arise from machining and assembling of mechanical parts. Non-geometric errors come from joint angle deviations caused by inaccurate encoder readings, mechanical clearances, compliance, backlash, and link deflection due to loading.
Most industrial robots use gear trains for power transmission to allow actuators to be located in some desirable positions. Gear trains are also used for torque amplification. Backlash is a persistent problem in such machines due to tooth clearances provided for prevention of jamming of gear teeth due to manufacturing errors or thermal expansion. Backlash introduces discontinuity, uncertainty and impact in mechanical systems which, in turn, makes accurate control of a manipulator difficult. End-effector positioning accuracy is also compromised due to backlash. Precision gears, spring-loaded split gear assemblies, and precise mechanical adjustment are often used to overcome these difficulties. However, these techniques do not completely eliminate the backlash and can increase the cost of manufacturing and assembling. Therefore, reducing or eliminating the backlash problem is urgently needed.
Prior anti-backlash techniques using an active actuator technique using unidirectional drives have included U.S. Pat. No. 3,512,425 where two motors drive a load through separate trains of gears. The motors are unidirectionally operated, but drive torque of each motor is always of an opposite sense so as to eliminate backlash. Such a system is limited to a single DOF with no mention of multiple degrees of freedom that would make it useful for robotic operations where multiple degrees of freedom are required.